The present invention relates to a liquid crystal display comprising MIM elements composed of metal-insulator-metal structures or metal-insulator-transparent and electrical conductor structures, and a method of manufacturing the same.
Along with an advance in commercial application of liquid crystal displays, liquid crystal displays of an active matrix type capable of displaying images of excellent quality have come by now to occupy a position in the mainstream of the market.
The active matrix type liquid crystal display described above comprises thin-film transistors (TFTs), diodes, or nonlinear resistance elements of a metal-insulator-metal (referred to hereinafter as xe2x80x9cMIMxe2x80x9d) structure composed of three layers consisting of metal-insulator-metal or metal-insulator-transparent and electrical conductor, as switching elements for each of liquid crystal display electrodes for displaying images.
The MIM elements described above is generally composed of a Taxe2x80x94Ta2O5xe2x80x94Cr or Taxe2x80x94Ta2O5-ITO structure. Herein, Ta refers to a tantalum film, Ta2O5 a tantalum oxide film, Cr a chromium film, and ITO an indium tin oxide film.
With a liquid crystal display using MIM elements, images are displayed by switching on and off a liquid crystal layer connected in series with the MIM elements by taking advantage of a nonlinear voltage-current characteristic of the MIM elements.
Now referring to FIGS. 29 to 32, the structure of a conventional liquid crystal display panel having nonlinear resistance elements composed of the Taxe2x80x94Ta2O5-ITO structure is described hereafter.
As shown clearly in FIG. 32, the MIM element comprises a tantalum (Ta) film as a lower electrode 103 formed on a first substrate 102, a tantalum oxide (Ta2O5) film as an insulation film 104 formed on the lower electrode, and a transparent and electrically conductive film 10 composed of an indium tin oxide (ITO) film as an upper electrode 105 formed on the insulation film, all these films together constituting a nonlinear resistance element.
In addition, the MIM element is provided with a display electrode 106 composed of an indium tin oxide film. Data signals dependent on the contents of display are applied on the display electrode 106 via the nonlinear resistance element by a signal electrode 107 composed of a tantalum film and a tantalum oxide film.
This liquid crystal display is provided with the first substrate 102 on which the nonlinear resistance elements are formed and a second substrate 109 (refer to FIG. 29) having opposite electrodes 110 (as indicated by phantom lines in FIG. 30) formed in such a way as to face the display electrodes 106 formed on the first substrate 102.
After applying liquid crystal-molecular alignment treatment to the surfaces of the first substrate 102 and the second substrate 109, the two substrates are bonded together with a sealing portion 108 such that the surfaces of the both substrates face each other at a predetermined spacing, and liquid crystals are sealed in a gap formed therebetween, thus forming a liquid crystal display. A region surrounded by a phantom line 118 as indicated in FIG. 29 and a solid line 118 as indicated in FIG. 30 represents a display region of the liquid crystal display.
However, the liquid crystal display having the conventional nonlinear resistance elements described above poses a problem of an after-image phenomenon occurring when an image displayed is changed in the course of driving the liquid crystal display.
Referring to FIG. 33, the after-image phenomenon is described. Herein, the liquid crystal display is assumed to display images in xe2x80x9cnormally whitexe2x80x9d mode.
FIG. 33 indicates variation in transmissivity of light when an applied voltage for a random pixel is varied for every 5 minutes . Specifically, a voltage (VI) for providing a display of 50% transmissivity is applied for first 5 minutes (unselect period: T1), then a voltage (V2) for providing a display of 10% transmissivity is applied for another 5 minutes (select period: T2), and further a voltage (V3) at the same level as that of the voltage (V1) applied for the first unselect period T1 is applied for yet another 5 minutes (unselect period: T3).
The after-image phenomenon is a phenomenon wherein a difference (xcex94T) in transmissivity between the unselect period T1 and the unselect period T3 develops although the voltages applied for respective periods are equal. With the liquid crystal display described above, the difference xcex94T in transmissivity was found to be 5%.
The occurrence of the after-image phenomenon results in the display of an image with its contents different from those of an originally intended image.
Therefore, an image sticking phenomenon, that is, the after-image phenomenon degrades considerably the quality of images displayed by the liquid crystal display, posing a serious problem in commercial application thereof.
A primary cause for the occurrence of the after-image phenomenon is a d-c voltage component of a voltage applied on the liquid crystal layer when driving the liquid crystal display. Owing to the d-c voltage component, a polarization phenomenon of alignment layers used for aligning liquid crystal molecules in a predetermined direction and the degradation of liquid crystals themselves occurs, resulting in the occurrence of the after-image phenomenon.
FIG. 34 is a graph showing a current-voltage characteristic (I-V characteristic) of a non-linear resistance element composed of a xe2x80x9ctantalum film-tantalum oxide film-indium tin oxide filmxe2x80x9d structure according to a conventional structure.
As shown in the figure, variation in current value differs considerably depending on the polarity of an applied voltage, demonstrating an asymmetrical current-voltage characteristic with respect to a voltage at zero.
As a means for achieving an improvement on the asymmetrical current-voltage characteristic, it is conceivable to replace the indium tin oxide film composing the upper electrode 105 of nonlinear resistance elements with such a metal film as a chromium (Cr) film, a titanium (Ti) film or the like.
Such replacement of the indium tin oxide film with the chromium film or the titanium film in forming the upper electrode 105 can moderate to some extent the asymmetry of the current-voltage characteristic as shown in FIG. 34, but is still far from achieving a fully symmetrical current-voltage characteristic.
Further, an offset driving method is proposed to prevent the d-c voltage component from being applied on the liquid crystal layer through the nonlinear resistance elements having the asymmetrical current-voltage characteristic. The offset driving method is described hereafter with reference to FIG. 35.
As shown in FIG. 35, the offset driving method is a method of driving the liquid crystal display by varying voltages applied in a select period (Ts) and a hold period (Th), respectively, depending on the polarity of an electric field, that is, a (+) field or a (xe2x88x92) field so that the d-c voltage component will not be applied on the liquid crystal layer by compensating for the asymmetric characteristic of the element with a varying driving voltage.
Voltages applied in the select period (Ts) are denoted Va1 and Va2, and voltages applied in the hold period (Th) are Vb1 and Vb2.
With the offset driving method as shown in FIG. 35, the d-c voltage component of a voltage applied between the display electrodes 106 and the opposite electrodes 110, disposed facing each other, with the liquid crystal layer sandwiched therebetween can be reduced.
However, asymmetrical voltages, for example, Vb2 and Vb1 are applied on the signal electrodes 107 as shown in FIGS. 30 and 31, but symmetrical voltages are applied on the liquid crystal layer. Consequently, a voltage between the signal electrodes 107 on the first substrate 102 composing the MIM elements and the display electrodes 106 contains the d-c voltage component. Furthermore, the d-c voltage component occurs similarly between the opposite electrodes 110 and the signal electrodes 107.
As a result, with nonlinear resistance elements having the asymmetric current-voltage characteristic, it was impossible to reduce sufficiently the d-c voltage component impressed on the liquid crystal layer, eliminating the after-image phenomenon completely.
Therefore, it is an object of the present invention to provide an liquid crystal display capable of displaying images of excellent quality without the effect of the after-image phenomenon by reducing a d-c voltage component impressed on the liquid crystal layer in its nonlinear resistance elements having an asymmetrical current-voltage characteristic.
To achieve the aforesaid object, the present invention provides a method of manufacturing liquid crystal display and a liquid crystal device as follows.
According to the present invention, a method of manufacturing the liquid crystal display comprises the following steps:
A. a process of forming a metal film on a substrate, then forming a plurality of anodic oxidation electrodes, a common electrode connecting together the anodic oxidation electrodes, lower electrodes of nonlinear resistance elements, and connection portions connecting the lower electrodes with the anodic oxidation electrodes by patterning on the metal film by means of a photo etching method;
B. a process of forming an insulation film by means of an anodic oxidation method applied to each of the anodic oxidation electrodes, the connection portions, and the lower electrodes, joined integrally with the common electrode, using the common electrode as an anode;
C. a process of forming a transparent and electrically conductive film on the insulation film and the substrate, then forming display electrodes on the substrate such that each of the display electrodes is provided with an overlapping portion covering a part of each of the connection portions, and forming a signal electrode on each of the anodic oxidation electrodes such that a gap is provided between each of the signal electrodes and each of the lower electrodes, then forming a first upper electrode connected with each of the single electrodes and a second upper electrode connected with each of the display electrodes, on each of the lower electrodes by patterning on the transparent and electrically conductive film by means of the photo etching method;
D. a process of forming a photosensitive resin in a region covering each of the lower electrodes, the first and the second upper electrodes; and
E. a process of etching the metal film and each of the connection portions, having a structure of laminated layers composed of the metal film and the insulation film of the anodic oxidation film formed on the metal film, completely down to the surface of the substrate by means of the etching method using the photosensitive resin, the display electrodes, and the signal electrodes as etching masks such that each of the connection portions automatically matches a plurality of sides of the display electrodes and the signal electrodes, separating the anodic oxidation electrodes disposed underneath the signal electrodes, overlapping portions of each of the connecting portions, disposed underneath each of the display electrodes, and the lower electrodes from each other such that each of the lower electrodes is isolated and formed in a shape resembling an island, forming a first nonlinear resistance element and a second nonlinear resistance element composed of each of the lower electrodes, the insulation film and the first and second upper electrodes, respectively, and electrically isolating the anodic oxidation electrodes form each other by removing the common electrode connecting the plurality of the anodic oxidation electrodes with each other by means of the etching method using the signal electrodes as etching masks.
In the step C, the display electrodes are formed such that each of the display electrodes is provided with overlapping portions covering parts of each of the connecting portions connecting the lower electrode of the nonlinear resistance element for a pixel adjacent to the relevant pixel with the anodic oxidation electrode.
The steps C, D, and E described as above may be replaced with the following steps:
a process of forming a metal film on the insulation film and the substrate which were formed in step B, then forming signal electrodes on the anodic oxidation electrodes, and forming a first upper electrode connected with each of the signal electrodes and a second upper electrode having a paid on each of the lower electrodes by patterning on the metal film by mans of the photo-etching method;
a process of forming a transparent and electrically conductive film on the insulation film and the substrate including the pad surface, then forming display electrodes electrically isolated from the signal electrodes and the lower electrodes such that each of the display electrodes is provided with an overlapping portion covering a part of each of the connection portions and the pad by patterning on the transparent and electrically conductive film by the photo-etching method;
a process of forming a photosensitive resin in a region covering each of the lower electrodes, the first and the second upper electrodes; and
a process of etching the metal film and each of the connection portions, having a structure of laminated layers composed of the metal film and the insulating film of the anodic oxidation film formed on the metal film, completely down to the surface of the substrate by means of the etching method using the photosensitive resin, the display electrodes, and the signal electrodes, each made of different material, as etching masks such that each of the connection portions automatically matches a plurality of sides of the display electrodes and the signal electrodes, separating the anodic oxidation electrodes disposed underneath the signal electrodes, overlapping portions of each of the connection portions, disposed underneath each of the display electrodes, and the lower electrodes from each other such that each of the lower electrodes is isolated and formed in a shape resembling an island, forming a first nonlinear resistance element and a second nonlinear resistance element composed of each of the lower electrodes, the insulation film and the first and second upper electrodes, respectively, and electrically isolating the anodic oxidation electrodes from each other by removing the common electrode connecting the plurality of the anodic oxidation electrodes with each other by means of the etching method using the signal electrodes as etching masks.
The process of forming a photosensitive resin, i a region covering each of the lower electrodes and the first and second upper electrodes, and the following process thereto may be replaced with the following steps:
a process of forming a photosensitive resin in a region covering about half of the first upper electrode and the second upper electrode, respectively, on the side facing each other, on each of the lower electrodes, and the surface of each of the lower electrodes therebetween; and
a process of etching each of the connection portions, having a structure of laminated layers composed of the metal film and the insulation film of the anodic oxidation film formed on the metal film, and each of the lower electrodes completely down to the surface of the substrate by means of the etching method using the photosensitive resin, the display electrodes, the first upper electrode, the second upper electrode, and the signal electrodes, all of which are made of different materials, as etching masks such that each of the connection portions automatically matches with a plurality of sides of the display electrodes and the signal electrodes, separating the anodic oxidation electrodes disposed underneath the signal electrodes, overlapping portions of each of the connection portions disposed underneath each of the display electrodes, and the lower electrodes from each other such that each of the lower electrodes automatically matches an external side of the first upper electrode and the second upper electrode, respectively, forming a first nonlinear resistance element and a second nonlinear resistance element composed of each of the lower electrodes, the insulation film and the first and second upper electrodes, respectively, and electrically isolating the anodic oxidation electrodes from each other by removing the common electrode connecting the plurality of the anodic oxidation electrodes with each other by means of the etching method using the signal electrodes as etching masks.
In the steps of the method of manufacturing the liquid crystal display, forming a metal film on the substrate, and forming a plurality of anodic oxidation electrodes, a common electrode connecting together the anodic oxidation electrodes, lower electrodes of nonlinear resistance elements, and connection portions connecting each of the lower electrodes with each of the anodic oxidation electrodes by patterning on the metal film by means of the photo-etching method,
each of the connection portions is formed integrally with the anodic oxidation electrode formed under the signal electrode in a row or column different from that for a signal electrode connected with the nonlinear resistance element for the relevant pixel; and
in the step of forming the display electrodes, the display electrodes are formed such that each of the display electrodes is provided with overlapping portions covering parts of each of the connection portions, connected with the lower electrode of the nonlinear resistance element for a pixel adjacent to the relevant pixel.
In the steps A to E of the method of manufacturing the liquid crystal display, the steps D and E may e replaced with the following steps:
a process of forming an overcoating insulation film on the entire surface of the substrate, after steps A to C, including the surfaces of all the electrodes and the connection portions formed thereon, after the aforesaid processes, and forming openings by the photo etching method in the overcoating insulation film in regions covering the overlapping portions where the display electrodes are partially overlapped with the connection portions, and parts of the connection portions, in regions protruding from the display electrodes; and
a process of etching each of the connection portions, having a structure of laminated layers composed of the metal film and the insulation film of the anodic oxidation film formed on the metal film, completely down to the surface of the substrate by means of the etching method using the overcoating insulation film, the display electrode inside the opening in the overcoating insulation film and the signal electrode as etching masks such that each of the connection portions automatically matches with a plurality of sides of the display electrode and the signal electrode, separating the anodic oxidation electrode disposed underneath the signal electrode, overlapping portions of each of the connection portions disposed underneath each of the display electrodes, and the lower electrodes form each other such that each of the lower electrodes is isolated and formed in an island-like shape, and forming a first nonlinear resistance element and a second nonlinear resistance element composed of the aforesaid lower electrode, the insulation film and the second upper electrodes, respectively.
By the method described above, it is possible to manufacture efficiently a liquid crystal display, having a pair of nonlinear resistance elements in good symmetry per pixel and having excellent quality of image without the effect of the after-image phenomenon.
In addition, the present invention provides another liquid crystal display described hereinafter.
The liquid crystal display provided with display electrodes disposed in a matrix configuration on a substrate, each of the display electrodes constituting a pixel, has the following constitution.
The liquid crystal display comprises an anodic oxidation electrode and lower electrodes each resembling an island in shape, both of which are composed of a metal film and formed on a substrate, an insulation film formed on the metal film, two upper electrodes composed of a transparent and electrically conductive film formed on each of the lower electrodes with the insulation film interposed in-between, display electrodes composed of a transparent and electrically conductive film, and signal electrodes composed of a metal film or a metal film and a transparent and electrically conductive film.
The two upper electrodes so disposed as to cross the lower electrode, the insulation film, and the lower electrode constitute two nonlinear resistance elements. One of the two upper electrodes, constituting one of the nonlinear resistance elements, is connected with one of the signal electrodes while the other is connected with one of the display electrodes.
The display electrodes constituting the pixels consist of two types of display electrodes, one type provided thereunder with an overlapping portion having a remaining double layer film, consisting of the same metal film as that for the lower electrodes and the insulation film, and the other type not provided thereunder with the overlapping portion.
In the case of a liquid crystal display wherein a plurality of display electrodes constitute one pixel, the display electrodes for the pixel consist of one display electrode provided thereunder with overlapping portions having a remaining double layer film, consisting of the same metal film as that for the lower electrodes and the insulation film, and the other display electrode not provided thereunder with the overlapping portion.
Or preferably, the display electrodes for constituting one pixel may consist of one display electrode provided thereunder with the overlapping portion having a remaining double layer film, consisting of the same metal film as that for the lower electrodes and the insulation film, and the other display electrode not provided thereunder with the overlapping portion, and furthermore, the nonlinear resistance elements of a plurality of display electrodes for constituting one pixel may be disposed to converge around a focal point.
Also, the present invention provides a liquid crystal display comprising an anodic oxidation electrode and lower electrodes, both of which are composed of a metal film and formed on a substrate, an insulation film formed on the surface of the metal film, upper electrodes formed on each of the lower electrodes with the insulation film interposed in-between, display electrodes connected with the upper electrodes, and signal electrodes composed of the lower electrode or the lower electrode and the insulation film.
And the upper electrodes so disposed as to cross the lower electrode, the insulation film, and the lower electrode constitute nonlinear resistance elements, and the anodic oxidation electrode or the anodic oxidation electrode and part of the insulation film are provided with overlapping portions kept intact under each of the display electrodes.
It is desirable that one of the two upper electrodes, constituting one of the nonlinear resistance elements, is connected with one of the signal electrodes while the other is connected with one of the display electrodes.
It is important to reduce an area ratio of the peripheral region of the display electrodes, not utilized for displaying images, to the display electrodes thereof to improve the quality of images. Also, in the case of a high density liquid crystal display, it is important to minimize the area of the peripheral region thereof.
For this reason, the display electrodes having the double layer films consisting of the metal film and the insulation film thereunder and the display electrodes not having the same can be provided by concentrating the nonlinear resistance elements, resulting in reduction of the area occupied by the connection portions. The connection portions being the areas shielding light, brighter display can be obtained in this way.
Furthermore, in the case of a liquid crystal display wherein a plurality of display electrodes constitute one pixel, the display electrodes can be made best use of by providing fewer number of the display electrodes having the double layer film thereunder than that of the display electrodes not having the same by concentrating the nonlinear resistance elements because the area of the connection portions shielding light is thus reduced.
In the case of a high density liquid crystal display provided with pixels at a small pitch or a large-sized liquid crystal display, the anodic oxidation electrode has limitations in its width. Accordingly, the anodic oxidation electrode with a large width is used until the anodic oxidation process is completed, and then a part thereof is removed after the display electrodes are formed. As this will enable the anodic oxidation electrode to have a large width, the insulation film required in the high density liquid crystal display or the large-sized liquid crystal display can be formed uniformly in a short time.
Furthermore, by using a part of the anodic oxidation electrode and the display electrode as etching masks when splitting the connection portion between the anodic oxidation electrode and the lower electrode resembling an island in shape, the insulation film can be formed uniformly in a short time without increasing the number of processing steps.
Also, the overlapping portions of the anodic oxidation electrode, remaining intact under the display electrodes, can then be used for a part of black matrices, contributing to an improvement on accuracy in aligning the black matrices with the display electrodes.